1. Field of the Invention
The present invention relates to an electron beam length-measurement apparatus and a measurement method for measuring a length of a predetermined portion in an object by using an electron beam.
2. Description of the Related Art
An optical length-measurement apparatus has been conventionally known that obtains an image of an object for which a length-measurement is to be performed by means of an optical microscope and measures a length of a predetermined portion in the object to be measured. In the electron length-measurement apparatus, the length measurement is performed for a GMR (Giant Magneto Resistive) head device as the object in some cases.
FIG. 1 schematically shows a structure of an exemplary GMR head device. The GMR head device includes a writing magnetic pole 200 for recording information onto a magnetic recording medium, a reading sensor 206 for reading information recorded onto the recording information medium, an upper shield layer 202 and a lower shield layer 204. In a case of the length measurement for the GMR head device, a width of the writing magnetic pole 200, a width of the reading sensor 206, and a length related to a relative position between the writing magnetic pole 200 and the reading sensor 206 is measured.
In recent years, a pattern in the GMR head device, such as a magnetic pole pattern, has become finer and therefore the length-measurement by the optical length-measurement apparatus has become difficult. Thus, an electron beam length-measurement apparatus that performs the length-measurement by using an electron beam has attracted attention.
FIGS. 2A and 2B are diagrams explaining a fine alignment, the length measurement and a focus analysis in the conventional electron beam length-measurement apparatus. Please note that the term “fine alignment” means an adjustment for realizing appropriate irradiation of the object to be measured with the electron beam in the present application.
In the conventional electron beam length-measurement apparatus, each of a plurality of positions (525 lines in FIG. 2A) determined in advance on the object to be measured is scanned with the electron beam a plurality of times (64 times in FIG. 2A), as shown in FIG. 2A. The fine alignment or the length measurement for a predetermined portion of the object is performed based on an image formed by secondary electrons detected by the above scanning and/or a manner, so-called a “line profiling”, in which the secondary electron amount and the backscattered electron amount detected during the scanning with the electron beam of each of the predetermined positions is changed.
In addition, as shown in FIG. 2B, in the conventional electron beam length-measurement apparatus, the focus position adjustment for the object is performed a plurality of times (4 times in FIG. 2B). Then, at each of the plurality of positions to which the focus position is adjusted, scanning with the electron beam is performed for a plurality of positions (525 lines in FIG. 2B) determined in advance for a plurality of times (64 times in FIG. 2B). The focus analysis is then performed based on contrast of the secondary electron image detected at each of the focus positions.
The electron beam length-measurement apparatus is required to measure the length of the predetermined portion of the object to be measured with high accuracy at high speed. Moreover, when the object to be measured is irradiated with the electron beam, the object may be contaminated. Thus, it is necessary to prevent contamination on the object to be measured.